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Saturday, August 04, 2012

Semiconductor Basics

Diode Vs Resistors

If Resistors are the most basic passive component in electrical or electronic circuits, then we have to consider the Signal Diode as being the most basic "Active" component.

However, unlike a resistor, a diode does not behave linearly with respect to the applied voltage as it has an exponential I-V relationship and therefore can not be described simply by using Ohm's law as we do for resistors.

Diodes are unidirectional semiconductor devices that will only allow current to flow through them in one direction only, acting more like a one way electrical valve, (Forward Biased Condition).
But, before we have a look at how signal or power diodes work we first need to understand their basic construction and concept.

Construction of Diode

Diodes are made from a single piece of Semiconductor material which has a positive "P-region" at one end and a negative "N-region" at the other, and which has a resistivity value somewhere between that of a conductor and an insulator.
But what is a "Semiconductor" material?, firstly let's look at what makes something either a Conductor or an Insulator.

Resistivity

The electrical Resistance of an electrical or electronic component or device is generally defined as being the ratio of the voltage difference across it to the current flowing through it, basic Ohm´s Law principals.

The problem with using resistance as a measurement is that it depends very much on the physical size of the material being measured as well as the material out of which it is made.

For example, If we were to increase the length of the material (making it longer) its resistance would also increase. Likewise, if we increased its diameter (making it fatter) its resistance would then decrease.

So we want to be able to define the material in such a way as to indicate its ability to either conduct or oppose the flow of electrical current through it no matter what its size or shape happens to be.

The quantity that is used to indicate this specific resistance is called Resistivity and is given the Greek symbol of ρ, (Rho). Resistivity is measured in Ohm-metres, ( Ω-m ) and is the inverse to conductivity.

If the resistivity of various materials is compared, they can be classified into three main groups, Conductors, Insulators and Semi-conductors as shown below.

Material Properties

Conductors

From above we now know that Conductors are materials that have a low value of resistivity allowing them to easily pass an electrical current due to there being plenty of free electrons floating about within their basic atom structure.

When a positive voltage potential is applied to the material these "free electrons" leave their parent atom and travel together through the material forming an electron drift.

Examples of good conductors are generally metals such as Copper, Aluminium, Silver or non metals such as Carbon because these materials have very few electrons in their outer "Valence Shell" or ring, resulting in them being easily knocked out of the atom's orbit.

This allows them to flow freely through the material until they join up with other atoms, producing a "Domino Effect" through the material thereby creating an electrical current.

Generally speaking, most metals are good conductors of electricity, as they have very small resistance values, usually in the region of micro-ohms per metre with the resistivity of conductors increasing with temperature because metals are also generally good conductors of heat.

Insulators

Insulators on the other hand are the exact opposite of conductors.

They are made of materials, generally non-metals, that have very few or no "free electrons" floating about within their basic atom structure because the electrons in the outer valence shell are strongly attracted by the positively charged inner nucleus.

So if a potential voltage is applied to the material no current will flow as there are no electrons to move and which gives these materials their insulating properties.

Insulators also have very high resistances, millions of ohms per metre, and are generally not affected by normal temperature changes (although at very high temperatures wood becomes charcoal and changes from an insulator to a conductor). Examples of good insulators are marble, fused quartz, p.v.c. plastics, rubber etc.

Insulators play a very important role within electrical and electronic circuits, because without them electrical circuits would short together and not work.

For example, insulators made of glass or porcelain are used for insulating and supporting overhead transmission cables while epoxy-glass resin materials are used to make printed circuit boards, PCB's etc.

Semiconductor Basics

Semiconductors materials such as silicon (Si), germanium (Ge) and gallium arsenide (GaAs), have electrical properties somewhere in the middle, between those of a "conductor" and an "insulator".

They are not good conductors nor good insulators (hence their name "semi"-conductors). They have very few "fee electrons" because their atoms are closely grouped together in a crystalline pattern called a "crystal lattice".

However, their ability to conduct electricity can be greatly improved by adding certain "impurities" to this crystalline structure thereby, producing more free electrons than holes or vice versa.

By controlling the amount of impurities added to the semiconductor material it is possible to control its conductivity.

These impurities are called donors or acceptors depending on whether they produce electrons or holes respectively.

This process of adding impurity atoms to semiconductor atoms (the order of 1 impurity atom per 10 million (or more) atoms of the semiconductor) is called Doping.

Silicon
The most commonly used semiconductor material by far is silicon.

Silicon has four valence electrons in its outermost shell which it shares with its neighboring silicon atoms to form full orbital's of eight electrons.

The structure of the bond between the two silicon atoms is such that each atom shares one electron with its neighbor making the bond very stable.

As there are very few free electrons available to move around the silicon crystal, crystals of pure silicon (or germanium) are therefore good insulators, or at the very least very high value resistors.

Silicon atoms are arranged in a definite symmetrical pattern making them a crystalline solid structure.

A crystal of pure silica (silicon dioxide or glass) is generally said to be an intrinsic crystal (it has no impurities) and therefore has no free electrons.

But simply connecting a silicon crystal to a battery supply is not enough to extract an electric current from it. To do that we need to create a "positive" and a "negative" pole within the silicon allowing electrons and therefore electric current to flow out of the silicon. These poles are created by doping the silicon with certain impurities.

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